Paroxysmal nocturnal hemoglobinuria (PNH) is a chronic hemolytic disorder characterized by the loss of membrane proteins that protect erythrocytes from complement mediated lysis. It is associated with a high risk of major thrombotic events, most commonly thrombosis of large intraabdominal veins. Most patients who die of their disease die of thrombosis. While the cause of this predisposition is unknown, we have recently demonstrated abnormal levels of platelet activation in patients with PNH. We have a protocol in place for treating patients with PNH with low dose oral aspirin. This protocol is designed to detect in vitro changes in platelet activation before and after aspirin administration. PNH is a rare disorder, and the protocol involves several visits to the National Institutes of Health campus over a month (platelet studies must be performed on fresh samples). Because of the difficulty of recruiting patients to this protocol, we are amending it to make it more convenient for patients.